Binary-Phase Acetonitrile and Water Aerosols: Infrared Studies and Theoretical Simulation at Titan Atmosphere Conditions

Acetonitrile (CH<sub>3</sub>CN) and water (H<sub>2</sub>O) ice particles were generated within a collisional cooling cell coupled to the Australian Synchrotron light source. The evolution of the aerosols was tracked by infrared spectroscopy compiled over the 4000–50 cm<sup>–1</sup> region. Gas pressure and temperature conditions were varied to replicate the lower altitudes of the Titan atmosphere allowing for comparison to far-infrared features detected by the Cassini–Huygens spacecraft. The experimental spectra show that CH<sub>3</sub>CN and H<sub>2</sub>O particles are microheterogeneous in composition and spherical in shape. CH<sub>3</sub>CN lattice bands display temperature-dependent shifts in frequency, implying that pure β-phase is present in the mixed particles. In addition, a red shift identified for the CN fundamental stretching mode indicates dipole–dipole and π-electron side-directed hydrogen bond coupling between segregated CH<sub>3</sub>CN and H<sub>2</sub>O phases exclusively at the grain interface. Discrete dipole approximation theory was implemented to evaluate various cluster architectures where segregated domains of pure CH<sub>3</sub>CN and H<sub>2</sub>O ices provided the best fit to experiment; confirming the infrared findings. Otherwise, simulations of competing architectures, such as core–shell and cubic shaped particles, did not provide convincing comparison to the aerosol spectra. We conclude that the far-infrared profiles for mixed CH<sub>3</sub>CN–H<sub>2</sub>O systems do not present as likely carriers for the unassigned 220 cm<sup>–1</sup> “haystack” feature that has been identified in Titan’s lower atmosphere.